This invention generally relates to a rendezvous maneuver for approaching a rendezvous spacecraft to a target spacecraft, and more particularly to a retry/recovery method in such a rendezvous maneuver for restoring the rendezvous spacecraft to a reference trajectory for approach to the target spacecraft in case of abnormality.
To achieve a rendezvous necessary for docking of spacecrafts or the like, trajectory control (called a rendezvous maneuver) of a rendezvous spacecraft (called a chaser) is required. Generally, a rendezvous approach of a rendezvous spacecraft to a target spacecraft (called a target) is performed by utilizing a difference in period between the two spacecrafts due to a difference in altitude between their orbits. For example, the rendezvous spacecraft approaches the target spacecraft through a trajectory which has a lower altitude and a shorter period (a greater angular velocity) than that of the target spacecraft. This preset approach trajectory is called a reference trajectory.
FIGS. 1A to 1C are views showing typical examples of the reference trajectory for approach in the rendezvous maneuver. Each of these drawings depicts an approach pattern of the rendezvous spacecraft to the target spacecraft in a target-centered, rotating coordinate-system where the origin represents the target spacecraft, the X-axis represents a distance in the direction of phase, and the Y-axis represents a distance in the direction of altitude. In the rendezvous maneuver, the rendezvous spacecraft is accelerated or decelerated at the apsis (apogee or perigee) in the direction of phase (the direction of X-axis) so that the apsis altitude of the rendezvous spacecraft gradually approaches the altitude of the target spacecraft. For example, acceleration at the apogee increases the altitude of the perigee, whereas acceleration at the perigee increases the altitude of the apogee.
FIG. 1A depicts the reference trajectory in the case where the apogee of a rendezvous spacecraft S.sub.R is set to the altitude H.sub.T of a target spacecraft S.sub.T and the perigee altitude of the rendezvous spacecraft is gradually increased in the order of a.sub.1, a.sub.2, a.sub.3, . . . as shown. This technique is called an A technique. FIG. 1B depicts the reference trajectory in the case where the apogee and perigee altitudes of the rendezvous spacecraft S.sub.R are alternately approached to the altitude H.sub.T of a target spacecraft S.sub.T. In other words, FIG. 1B depicts the reference trajectory in the case where a circular orbit of the rendezvous spacecraft S.sub.R lower than the altitude H.sub.T of the target spacecraft S.sub.T is approached to that of the target spacecraft S.sub.T. This technique is called a B technique. In these cases, the rendezvous spacecraft S.sub.R is finally approached to the target spacecraft S.sub.T using a navigation sensor such as a radar. FIG. 1C depicts the reference trajectory based on a so-called C technique where the A and B techniques are combined. In this case, the rendezvous spacecraft is accelerated in the direction of speed at points P.sub.1 to P.sub.5, TI and TF. Note that TF represents a final point of the trajectory of the rendezvous spacecraft.
While the rendezvous of spacecrafts are intended for in-orbit service such as resupply and/or withdrawal of materials, the launching cost of rendezvous spacecrafts is very expensive. When approaching the rendezvous spacecraft to the target spacecraft, therefore, if the rendezvous spacecraft cannot be injected into the reference trajectory for approach by reasons such as any trouble in a thruster or a large maneuver error, it is required to promptly perform a retry/recovery for the improved rate of mission achievement.
However, the retry/recovery of the rendezvous maneuver accompanies the following problems:
(1) If the retry is started after the rendezvous spacecraft has approached the target spacecraft closer than scheduled, there is a risk that the rendezvous spacecraft may enter a collision course with respect to the target spacecraft.
(2) If the retry is not performed promptly, the rendezvous spacecraft may pass over and go ahead of the target spacecraft, making the recovery to the reference trajectory extremely difficult.
(3) Once any abnormality happens, it is hard to make a judgment on whether or not guidance and control will be normally performed after that.
To date, the retry/recovery has been required to be designed in a very complex manner, taking into account the above problems.